Asymmetric Cobalt‐Catalyzed Regioselective Hydrosilylation/Cyclization of 1,6‐Enynes

Abstract

We report an enantioselective cobalt-catalyzed hydrosilylation/cyclization reaction of 1,6-enynes with secondary and tertiary hydrosilanes with a catalyst generated in situ from the combination of Co(acac) 2 and ( R , S p )-Josiphos. Under identified conditions, a wide range of oxygen-, nitrogen-, and carbon-tethered 1,6-enynes reacted with Ph 2 SiH 2 , (EtO) 3 SiH, or (RO) 2 MeSiH to afford the corresponding chiral organosilane products in high yields with excellent enantioselectivity (up to >99% ee). In addition, this cobalt-catalyzed hydrosilylation/cyclization also occurred with prochiral secondary hydrosilane PhMeSiH 2 to yield chiral alkylsilanes containing both carbon- and silicon-stereogenic centers with excellent enantioselectivity, albeit with modest diastereoselectivity. The chiral organosilane products from this cobalt-catalyzed asymmetric hydrosilylation/cyclization could be converted to a variety of chiral five-membered heterocyclic compounds by stereospecific conversion of their C-Si and Si-H bonds without loss of enantiopurity. Mechanistic studies including deuterium-labeling, control, and competition experiments suggest a chiral cobalt hydride intermediate [( L* )Co-H], the chelation of 1,6-enynes to cobalt catalysts, and the regioselectivity controlled by the relative reactivity of the alkyne and alkene moieties toward hydrocobaltation.